Locating multiple wellbores

ABSTRACT

Methods and systems are described that can be used for locating conductive bodies such as wellbore casing or piping disposed below the earth&#39;s surface. An electrical current can be excited in a conductive body in each wellbore in a given area to produce a magnetic field, and the magnetic field can be detected by a ranging tool. Location and current parameters can be determined for an estimated number of wellbores producing non-negligible contributions to the magnetic field, and the estimated number of wellbores can be adjusted until the number of contributing wellbores is determined. Location solutions can be returned for each of the contributing wellbores, and the location solutions can be employed to facilitate exploration of drilling applications such as well avoidance, well intersection and/or steam assisted gravity drainage (SAGD) steering operations.

BACKGROUND 1. Field of the Invention

The present disclosure relates generally to making measurements relatedto oil and gas exploration. More particularly, embodiments of thedisclosure relate to systems and methods for measuring electromagneticfields to detect a number of nearby wellbores, and to determine thelocations of the nearby wellbores.

2. Background

In drilling wells for oil and gas exploration, understanding thestructure and properties of the associated geological formation providesinformation to aid such exploration. In addition, drilling can beenhanced with systems and methods to detect conductive bodies below theearth's surface. The conductive bodies can include metal structures,e.g., casing and piping used in various down-hole operations, where thedetection of the metal structures can reveal subterranean positions ofexisting wellbores. For example, the positions of existing wellbores caninfluence drilling operations in crowded fields where legal or landrestrictions require wells to be drilled in close proximity with oneanother. In some instances, well avoidance is an important considerationin drilling in the crowded fields, and in some instances, wellintersection can be intentional, e.g., for the repair of a damagedwellbore.

In other instances, determining the position of existing wellbores canfacilitate drilling a wellbore in a predetermined relationship to theexisting wellbore. For example, in steam assisted gravity drainage(SAGD) applications, a wellbore is often drilled parallel to existingwellbores. Generally in SAGD applications, steam is used in conjunctionwith two spaced apart wellbores (an SAGD pair) to address the mobilityproblem of heavy oil in a formation. High temperature steam may beinjected into an upper wellbore (the injector), and used to heat theheavy oil to reduce its viscosity and thereby enhance the flow of theheavy oil into a lower wellbore (the producer) to enhance extraction ofthe heavy oil. Preferably, the injector and producer are drilled at adistance of only a few meters from one other, e.g., about 5 to about 15meters. If the injector is positioned too close to the producer, theproducer would be exposed to very high pressure and temperature andsteam would be directly communicated to the producer. If the injector ispositioned too far from the producer, the efficiency of the SAGD processis reduced. In order to assist in ensuring that the injector of an SAGDpair is positioned as desired relative to the producer wellbore, asurvey of the geologic formation is often conducted. These surveyingtechniques are traditionally referred to as “ranging.” Often, theseranging techniques can be frustrated by the proximity of multiple nearbywellbores, e.g., where it can be difficult to distinguish magneticfields contributed from a target wellbore from other nearby existingwellbores in a crowded field.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure is described in detail hereinafter on the basis ofembodiments represented in the accompanying figures, in which:

FIG. 1A is a cross-sectional schematic side-view of a system ofwellbores including first and second existing wellbores and a thirdwellbore having a drill string or other conveyance equipped with aranging tool disposed therein in accordance with one or more exemplaryembodiments of the disclosure;

FIG. 1B is a cross-sectional schematic top-view of the system ofwellbores of FIG. 1A illustrating a crowded field of existing wellboresproximate the first, second and third wellbores;

FIG. 2 is a schematic diagram of system for determining the number andlocations of multiple wellbores including receivers of the ranging toolof FIG. 1A in accordance with example embodiments of the disclosure;

FIG. 3 is a schematic view of the ranging tool and the first wellbore ofFIG. 1A illustrating various parameters of the first wellbore that aredeterminable by the system of FIG. 2;

FIG. 4 is a schematic view of the ranging tool and multiple wellbores ofthe network of FIG. 1A illustrating magnetic fields detectable by theranging tool;

FIGS. 5A and 5B are flowcharts illustrating operational procedures thatinclude locating multiple wellbores in accordance with exampleembodiments of the disclosure;

FIG. 6 is a graphical view of a mathematically simulated example of atwo-wellbore system illustrating simulated location curves for first andsecond wellbores and a simulated path of a ranging tool for determiningthe respective locations of the first and second wellbores using aninversion portion of the operational procedure illustrated in FIG. 5A;

FIG. 7 is a schematic view of a sensor arrangement for the ranging toolof FIG. 6;

FIG. 8 is a graphical view of the simulated two-wellbore system of FIG.6 illustrating positional data points calculated for the first andsecond wellbores overlying the respective simulated location curves; and

FIGS. 9A and 9B are graphical views illustrating data indicative of acurrent magnitude calculated for a simulated current of 1 Amp flowingrespectively through the first and second wellbores of FIG. 6.

DETAILED DESCRIPTION

In the interest of clarity, not all features of an actual implementationor method are described in this specification. Also, the “exemplary”embodiments described herein refer to examples of the present invention.In the development of any such actual embodiment, numerousimplementation-specific decisions may be made to achieve specific goals,which may vary from one implementation to another. Such wouldnevertheless be a routine undertaking for those of ordinary skill in theart having the benefit of this disclosure. Further aspects andadvantages of the various embodiments and related methods of theinvention will become apparent from consideration of the followingdescription and drawings.

The present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed. Further, spatiallyrelative terms, such as “below,” “upper,” “up-hole,” “down-hole,” andthe like, may be used herein for ease of description to describe oneelement or feature's relationship to another element(s) or feature(s) asillustrated in the figures. The spatially relative terms are intended toencompass different orientations of the apparatus in use or operation inaddition to the orientation depicted in the figures.

1. Description of Exemplary Embodiments

FIG. 1 illustrates a multi-wellbore system 10 including a first wellbore12 and a second wellbore 14 formed in a geologic formation “G.” In someexemplary embodiments, the geologic formation “G” is an oil sand orother heavy oil formation disposed at a relatively shallow depth, e.g.,70-600 m below a surface location “S.” The first and second wellbores12, 14 can be employed for recovery of heavy oil from the geologicformation “G” using processes such as SAGD operations. The systems andmethods, in accordance with the teaching herein, can be used to provideSAGD steering, and such SAGD steering can be applied to non-horizontalwells as well. Although the multi-wellbore system 10 is described hereinbelow as employed in conjunction with SAGD operations, it should beappreciated that aspects of disclosure may be practiced in conjunctionwith offshore ranging operations or other types of exploration as well.

The first and second wellbores 12, 14 include respective electricallyconductive bodies 12 a, 14 a therein. The electrically conductive bodies12 a, 14 a can include strings of casing pipe, production tubing orother elongate metallic media in which electrical currents can beexcited. As described in greater detail below, electrical currentsexcited in the electrically conductive bodies 12 a, 14 a in the firstand second wellbores 12, 14 can facilitate magnetic ranging processesfor drilling a third wellbore 16 along a path having a predeterminedrelationship with a path defined by the first wellbore 12. Thepredetermined relationship can include, e.g., a generally parallel,horizontal relationship to facilitate SAGD operations. By allowing twoor more wellbores 12, 16 to be positioned within extremely tighttolerances, magnetic ranging technology can help eliminate positionaluncertainty of conventional surveys and can provide more precisetolerance for SAGD wells. In other embodiments, the predeterminedrelationship can be an intersecting relationship where the thirdwellbore 16 intersects the first wellbore 12 at a predetermined and/ortrue depth, a lateral branching relationship where the third wellbore 16branches from the first wellbore 12 at a predetermined location.

The first wellbore 12 can be employed as the “producer” in a SAGD systemand the third wellbore 16 can be employed as the “injector” in a SAGDsystem. The third wellbore 16 is being drilled proximate the firstwellbore 12 at a distance “D” therefrom. In some example embodiments,the distance “D” can be in the range of about 5 to about 15 meters. Thethird wellbore 16 is being drilled with a drill string 18 having a drillbit 20 at a lower end thereof. A slant drilling rig 22 is erected at thesurface location “S” to support the drill string 18 at shallow angles,e.g., at angles in the range of about 15° to about 75° from horizontal.In some example embodiments, the slant drilling rig 22 can support thedrill string 18 at angles of between 0° and 90° (inclusive) fromhorizontal. Often slant drilling is employed to facilitate access to theshallow geologic formations “G” where heavy oils are entrapped and SAGDoperations are generally conducted.

A ranging tool 26 is carried by the drill string 18. The ranging tool 26can be employed to determine the location of the conductive body 12 awithin the first wellbore 12, thus permitting the drill bit 20 to besteered to maintain the third wellbore 16 at the distance “D” from thefirst wellbore 12, or at any spatial relationship therewith. Asdescribed in greater detail below, the ranging tool 26 is operable todetect a magnetic field generated by separate electric currentspropagated through electrically conductive bodies 12 a, 14 a, andmeasure wellbore parameters of both the first and second wellbores 12,14 to guide the drill string 18. In some example embodiments, theranging tool 26 can be deployed independently of a drill string 18. Forexample, the ranging tool 26 can be deployed into an additional existingwellbore (not shown) by wireline, coiled tubing or other recognizedmethods. Alternatively or additionally, the ranging tool 26 can bedeployed by moving the ranging tool 26 across the surface location “S.”

The electric currents propagated through the conductive bodies 12 a, 14a can be excited in any recognized manner, e.g., by using a down-holeelectrode source 30, or by directly coupling an electrically conductivebody 12 a to a current source 32 at the surface location “S.” In someexample embodiments, the currents can be remotely excited by coilantennas 34, which form a part of, or are carried by, the drill string18. In some embodiments, the electrical currents can be excited by thetransmitters 52 (FIG. 2) provided in the ranging tool 26.

Referring to FIG. 1B, in some example embodiments, the first wellbore 12in the multi-wellbore system 10 can have a plurality of “fishbone” orlateral wellbores 12′ branching therefrom. The lateral wellbores 12′ canpermit production from a wider expanse of the geologic formation “G”than through the first wellbore 12 alone. Similarly, the second wellbore14 can include lateral wellbore 14′ branching therefrom. Additionalwellbores 36 a through 36 c are drilled in the vicinity of the first andsecond wellbores 12, 14 and together with the respective lateralwellbores 36 a′ through 36 c′, the wellbores 12, 14 and 36 a through 36c can produce from a relatively large horizontal area of the geologicformation “G.” Each of the wellbores 12, 14 and 36 a through 36 c can bedrilled from a relatively small, common drilling area 40. The field ofwellbores defined by the wellbores 12, 14 and 36 a through 36 c can becharacterized as a crowded field, particularly near the common drillingarea 40. As the third wellbore 16 is drilled through the crowded field,e.g., from the drilling area (point A) to a terminal end (point B), thenumber of “nearby” wellbores frequently changes. As used herein, a“nearby wellbore” includes at least a wellbore having a current excitedtherein that produces a magnetic field detectable by a ranging tool 26.Each one of the wellbores 36 a through 36 c, and each of the respectivelateral wellbores 36 a′ through 36 c′ can also include an individualelectrically conductive body (not explicitly shown) therein that cangenerate a magnetic field detectable by the ranging tool 26. Thus,accurately interpreting the electromagnetic field received and detectedby the ranging tool 26 at any particular location can includedetermining a number of wellbores making a contribution to the receivedelectromagnetic field.

Referring to FIG. 2 and with continued reference to FIGS. 1A and 1B, asystem 50 for determining the number, locations and/or other wellboreparameters of multiple wellbores is illustrated. The system 50 caninclude transmitters 52 and receivers 54. In some example embodiments,the transmitters 52 can form a part of the ranging tool 26, and can becarried by a drill string 18 (FIG. 1A). The transmitters 52 can beoperable to generate an electromagnetic probe signal that causes currentto flow in the electrically conductive bodies 12 a, 14 a (FIG. 1A). Insome example embodiments, the transmitters 52 can include a focusedelectric dipole source operable to induce the electric current in theelectrically conductive body 16, which is exterior to the structure ofthe transmitters 52. In some example embodiments, the transmitters 52can be deployed independently of the ranging tool 26, e.g., by wirelineinto an adjacent wellbore 36 b, or can be mounted at stationary locationwith respect to the geologic formation “G.” As used herein, the term“transmitter” can include other such mechanisms for inducing theelectric currents in the electrically conductive bodies 12 a and 14 c(FIG. 1A) such as the electrode 30 and/or current source 32.

The receivers 54 may include any mechanism that detects, measures and/orcollects a magnetic field for processing. For example, coil antennas ormagnetometers can be used. The receivers 54 can be operable to measureor collect for processing an electromagnetic field emitted from one ormore electrically conductive bodies 16 (FIG. 1A), as a result of theprobe signal generated from the transmitters 52. The electricallyconductive bodies 12 a, 14 a, and the wellbores 12, 14, 36 a through 36c and 36 a′ through 36 c′ containing the electrically conductive bodies12 a, 14 a, can be detected from the magnetic field, and the appropriatelocations and other wellbore parameters can be determined therefrom.

The system 50 also includes a communications unit 56, which facilitatesinteraction among a system control center 58, a visualizing unit 60 andthe transmitters 52 and receivers 54. The communications unit 56 canprovide a bi-directional telemetry system for communication betweendown-hole components, e.g., the ranging tool 26, and components locatedat the surface location “S,” e.g., the visualizing unit 60.Communications unit 56 can employ combinations of wired communicationtechnologies and wireless technologies at frequencies that do notinterfere with on-going measurements made by the ranging tool 56. Thevisualizing unit 60 can include computer monitors, printers or otherdevices that permit an operator at the surface location “S” to monitordata generated by the system 50. The operator may monitor the data todetermine whether intervention into the system 50 is necessary.

The system control center 58 can be disposed within the ranging tool 26,at another down-hole location, or at the surface location “S,” and caninclude a computer with a processor and a non-transitory memoryassociated therewith. The system control center 58 can manage thegeneration of transmission signals, e.g., instructions signals for thetransmitters 52, and can manage the collection of received signals,e.g., data signals from the receivers 54, corresponding to thetransmission signals. The frequency at which the transmission signalsare provided can be controlled by the system control center 58. Thesystem control center 58 is also operably coupled to a data acquisitionunit 62 and a data processing unit 64. The data acquisition unit 62 caninclude analog to digital converters, or other mechanisms for convertingthe received signals into a form useable by the system control center 58and/or the data processing unit 64.

The data processing unit 64 can convert the received signals intoinformation indicating the number, position and directionalcharacteristics of wellbores in the vicinity of the ranging tool 26 asdescribed in greater detail below. This information can be provided tothe visualizing unit 60 for evaluation by an operator. In someembodiments, the data processing unit 64 may include a processor 64 aand a computer readable medium 64 b operably coupled thereto. Thecomputer readable medium 64 b can include a nonvolatile ornon-transitory memory with data and instructions that are accessible tothe processor 64 a and executable thereby. In one or more embodiments,the computer readable medium 64 b is pre-programmed with predeterminedthresholds Th^(field) and Th^(misfit), which, as described in greaterdetail below, can be at least partially dependent on characteristics ofthe ranging tool 26. The computer readable medium 64 b may also bepre-programmed with predetermined sequences of instructions forperforming various steps of procedures 100, 200 described below withreference to FIGS. 5A and 5B.

Referring to FIG. 3, parameters associated with a wellbore, which can bedetermined by the data processing unit 64 (FIG. 2) are illustrated.These parameters include position and direction parameters that canfully define the location of the wellbore in three-dimensional space.Only the first wellbore 12 is illustrated in FIG. 3, although, eachwellbore in the multi-wellbore system 10 can be defined by a similarparameter set.

In one or more embodiments, a wellbore parameter set may include avector parameter {right arrow over (I)}_(i), which represents a currentflowing through the wellbore 12 in an axial direction of the wellbore12, and a vector parameter {right arrow over (r)}_(i) representing anorthogonal distance between the wellbore 12 and a reference point “P”defined with respect to the ranging tool 26. The reference point “P” isillustrated at the center of coordinate system 66, and the vectorparameters {right arrow over (I)}_(i) and {right arrow over (r)}_(i) canbe directionally defined with respect to the coordinate system 66. Thesubscript “i” denotes an index number of the first wellbore 12, or moregenerally, any of the wellbores in a multi-wellbore system 10. The index“i” is an integer ranging from 1 to N^(pipe), where N^(pipe) is a numberof wellbores. The number of wellbores represented by N^(pipe) can be atotal number of wellbores in a multi-wellbore system 10, a number ofnearby wellbores that make a non-negligible contribution (defined ingreater detail below) to the magnetic field detected by the ranging tool26, or an estimated number of wells. Both of the vector parameters{right arrow over (I)}_(i) and {right arrow over (r)}_(i) have threedegrees of freedom, and thus, there are at least six variables or“unknown” parameters in a parameter set associated with the firstwellbore 12, and each of the N^(pipe) wells. The vector parameters{right arrow over (I)}_(i) and {right arrow over (r)}_(i) aredeterminable by the system 50, and other parameters such as azimuthangle and orientation of the first wellbore 12 can be determined fromthe vector parameters {right arrow over (I)}_(i) and {right arrow over(r)}_(i). For steering the ranging tool 26 relative to the firstwellbore 12, it is not necessarily required to determine the strength ofthe current propagated through the first wellbore 12, i.e., themagnitude of current vector {right arrow over (I)}_(i). However, asdescribed below, determining the current vector {right arrow over(I)}_(i) can facilitate determining the contribution of the firstwellbore 12 to a field detected by the ranging tool 26.

As illustrated in FIG. 4, a first magnetic field H₁ is induced by thefirst effective current I_(eff) ₁ flowing through the first wellbore 12.A second magnetic field H₂ is induced by a second effective currentI_(eff) ₂ flowing through the second wellbore 14. The ranging tool 26 islocated at within the third wellbore 16 at respective radial distancesR₁ and R₂ from the wellbores 12 and 14. The ranging tool 26 can includereceivers 54 in the form of tri-axial coil antennas, i.e., a set ofthree antennas whose magnetic moment vectors form an orthogonal systemspanning the entire field. Thus, the ranging tool 26 can be sensitive tothe fields H₁ and H₂, regardless of the orientation of the fields H₁ andH₂ with respect to the receivers 54. The combined magnetic fielddetected by the ranging tool 26 includes a contribution from each of thefields H₁ and H₂. Assuming that the wellbores 12 and 14 are infinitelylong, and assuming that the effective currents I_(eff) ₁ and I_(eff) ₂are constant, each of the magnetic fields H₁ and H₂ can be expressed byequation (1) below.

$\begin{matrix}{{\overset{\rightarrow}{H}\left( \overset{\rightarrow}{r} \right)} = {\frac{I_{eff}}{2\pi R}\overset{\hat{}}{\varphi}}} & (1)\end{matrix}$

In equation (1), {right arrow over (H)} denotes the magnetic field,{circumflex over (ϕ)} is the unit vector in an azimuthal direction, and{right arrow over (r)} is the position vector for the receivers 54 (FIG.2) of the ranging tool 26. Even though equation (1) is used as the basisof the mathematical formulation presented next, in some exemplaryembodiments, is possible to replace equation (1) with a more preciseversion of the equation by including the axial variations of the currenton a pipe, for example. From equation (1) it is seen that the magneticfield {right arrow over (H)} detected by the ranging tool 26 can beinversely proportional to the radial distances R₁ and R₂ between thewellbores 12 and 14 and the receivers 54. The assumptions regarding theinfinite lengths and constant currents have generally been demonstratedto yield sufficiently accurate approximations of the wellbore parametersfor steering purposes. In particular, these assumptions yield relativelyaccurate approximations of the wellbore parameters for wellbores nearestthe receivers 54 where the radial distances R₁ and R₂ are the smallestand where accuracy can be relatively significant for steering. Theformulation of the magnetic fields given in equation (1) is thusemployed throughout this specification. However, if differentdescriptions of the magnetic fields radiated from the wellbores areavailable; these descriptions may be employed without departing from thespirit and scope of the present disclosure.

When the ranging tool 26 is deployed, each receiver 54 can make distinctmeasurements of the total magnetic field {right arrow over (H)}. Anumber of receivers 54 provided on the ranging tool 26 can be designatedas N^(rec). In some exemplary embodiments, at least some of thereceivers 54 are single axis magnetometers, and in some exemplaryembodiments, at least some of the receivers 54 are tri-axial or othermulti-axial receivers. The number of receivers N^(rec) can designate thenumber of axes along which the receivers are disposed. For example,where four (4) tri-axial receivers 54 are provided on a ranging tool 26in various orientations, the number of receivers N^(rec) can be definedas twelve (12). Using the formulation for the magnetic fields given inequation (1) above, for each of the N^(rec) receivers 54, the magneticfield can be described by one of the N^(rec) equations illustrated byequations (2) through (3) below.

$\begin{matrix}{{{Hu_{1}} = {\sum\limits_{i = 1}^{N^{pipe}}{\frac{{\overset{->}{I}}_{i}}{2\pi {{{\overset{\rightarrow}{r}}_{i} - {\overset{\rightarrow}{r}}_{u1}}}}{\left( {\overset{\rightarrow}{I}i \times \left( {{\overset{\rightarrow}{r}}_{i} - {\overset{\rightarrow}{r}}_{u1}} \right)} \right) \cdot {\overset{\hat{}}{u}}_{1}}}}}\vdots} & (2) \\{{Hu}_{N^{rec}} = {\sum\limits_{i = 1}^{N^{pipe}}{\frac{\overset{->}{I_{i}}}{\left. {2\pi} \middle| {{\overset{\rightarrow}{r}}_{i} - {\overset{\rightarrow}{r}}_{u_{N}rec}} \right|}{\left( {\overset{\rightarrow}{J}i \times \left( {{\overset{\rightarrow}{r}}_{i} - {\overset{\rightarrow}{r}}_{u_{N^{rec}}}} \right)} \right) \cdot {\overset{\hat{}}{u}}_{N^{rec}}}}}} & (3)\end{matrix}$

In equations (2) through (3), Hu_(k) generally denotes the magneticfield measured by the k^(th) receiver 54 of the ranging tool 26, where kis an index number ranging from 1 to N^(rec). The variable û_(k)represents the unit vector in the direction that the k^(th) receiver 54is oriented to receive the magnetic field, e.g., the magnetic momentvector's direction for a magnetic dipole. The variable {right arrow over(r)}_(uk) is the position vector of the k^(th) receiver 54 with respectto the reference point “P.” These equations indicate that the magneticfield Hu_(k) detected by one of the receivers 54 will be a summation ofthe contributions made by each of the N^(pipe) currents induced inwellbores.

In addition to the N^(rec) equations (2) through (3), another system ofequations can be defined for the wellbore parameters using theassumption that the direction of the current induced in a wellbore willbe orthogonal to the position vector. As illustrated in the N^(pipe)equations in equations (4) through (5) below, the inner dot product ofthe two orthogonal vectors {right arrow over (I)}_(i) and {right arrowover (r)}_(i) will be zero.

$\begin{matrix}{{{{\overset{->}{I}}_{1} \cdot {\overset{\rightarrow}{r}}_{1}} = 0}\vdots} & (4) \\{{{\overset{\rightarrow}{I}}_{N^{pipe}} \cdot {\overset{\rightarrow}{r}}_{N^{pipe}}} = 0} & (5)\end{matrix}$

The total number of equations defined by equations 2 through 5 is(N^(pipe)+N^(rec)) the sum of the number of wellbores (N^(pipe)) and thenumber of receivers (N^(rec)). Since there are six “unknown” parametersassociated with each of the N^(pipe) wellbores as described above, thetotal number of unknown parameters, and thus the minimum number ofequations necessary for obtaining an unambiguous solution for theunknown parameters is 6×N^(pipe). Thus, to obtain an unambiguoussolution for the unknown wellbore parameters, the relation indicated inrelation (6) below should generally be met. It follows that generally,the number of wellbores N^(pipe) can be a function of the number ofreceivers N^(rec) provided, and a maximum number of wellbores that canbe determined unambiguously as illustrated in relation (7).

$\begin{matrix}{{N^{pipe} + N^{rec}} \geq {6 \times N^{pipe}}} & (6) \\{\left. \Rightarrow{N^{pipe} \leq \frac{N^{\tau ec}}{5}} \right. = N^{m\; {ax}}} & (7)\end{matrix}$

In some instances, where some additional information is available or canbe estimated, a fewer number of receivers N^(rec) can allow for agreater number of wellbores N^(pipe) to be located. For example, whereit is known, or it can be estimated, that each of the wellbores in amulti-wellbore system is parallel, the general relations (6) and (7) canbe reduced to relations (8) and (9) below.

$\begin{matrix}{{N^{pipe} + N^{rec}} \geq {{4 \times N^{pipe}} + 2}} & (8) \\{\left. \Rightarrow{N^{pipe} \leq \frac{\left( {N^{rec} - 2} \right)}{3}} \right. = N^{m\; {ax}}} & (9)\end{matrix}$

This reduction is possible where the direction of each of the currents{right arrow over (I)}_(i) is known to be the same for each wellborealthough the amplitude for each current {right arrow over (I)}_(i) maybe dissimilar. In some embodiments, this information can be known, e.g.,in a multi-wellbore system including a one or more parallel of SAGDpairs.

The number of wellbores N^(pipe) can include a number of wellbores thatproduce a non-negligible magnetic field at the ranging tool 26. In acrowded field, such as the field depicted in FIG. 1B, contributions froma large number of wellbores may be detected by the ranging tool 26.However, some of these contributions may be negligible. For example, theattenuation of the magnetic fields produced by currents induced inwellbores at large distances from the ranging tool 26 can producerelatively small contributions to the magnetic fields detected. Also,destructive interference can produce a cancellation effect on themagnetic fields than can contribute to relatively small contributions tothe magnetic fields detected. The definition of negligibility can bedependent on several system-dependent characteristics or requirementssuch as accuracy, noise floor, etc. A threshold Th^(field) can bepredetermined by one skilled in the art below which a particularcontribution can be considered negligible and removed from furtherconsideration as part of the multi-wellbore system.

Referring to FIG. 5A, and with continued reference to FIG. 2, anoperational procedure 100 can be employed for locating multiplewellbores using the data processing unit 64 of system 50 (FIG. 2).Preconditions for the procedure 100 can include that a current isinduced in each wellbore to be located, the ranging tool 26 is deployedat a first operational location, and a received magnetic field {rightarrow over (H)}^(rec) from the first operational location is detected bythe receivers 54 and communicated to the data processing unit 64, whichaccepts the received magnetic field {right arrow over (H)}^(rec) asinput. The operational procedure 100 begins at step 102 where thereceived magnetic field {right arrow over (H)}^(rec) is received by thedata processing unit and evaluated to determine if the magnitude or normof the received magnetic field {right arrow over (H)}^(rec) is greaterthan the predetermined field threshold Th^(field) for negligibility. Ifthe norm of the received magnetic field {right arrow over (H)}^(rec) isnot greater than the predetermined field threshold Th^(fieid), then itcan be determined that there are no nearby wellbores producing anon-negligible magnetic field. The procedure 100 proceeds to step 104where the procedure 100 may end. Optionally, at the end of the procedure100, the data processing unit 64 can provide a status to thevisualization unit 60, such that a user can be alerted that no nearbywellbores were located at the first operational position. The rangingtool 26 can then be moved to a second operational position, and theprocedure 100 can be repeated. The preconditions and post process returnto step 102 where another received magnetic field {right arrow over(H)}^(rec) can be evaluated.

Where it is determined at step 102 that the norm of the receivedmagnetic field {right arrow over (H)}^(rec) is greater than thepredetermined field threshold Th^(field), there may be nearby wellborescontributing to the received magnetic field {right arrow over(H)}^(rec). The procedure 100 proceeds to step 106 where an iterativeinversion process can be initiated to determine the number and locationsof the contributing wellbores. Initially, at step 106, an estimatednumber of contributing wellbores can be selected. Generally, an estimateof one (1) can be selected, although in some exemplary embodiments ahigher number of wellbores can be estimated, e.g., if it is known that aminimum number of wellbores are nearby. The variable N^(pipe) is set tothe initial estimate at step 106.

Next, the procedure 100 proceeds to step 108 where at least oneparameter set is calculated for the estimated number of wellboresN^(pipe) which would produce the received magnetic field {right arrowover (H)}^(rec). For example, when the estimated number of wellboresN^(pipe) is set to one (1), the parameter set for a single wellborewhich would produce the received magnetic field {right arrow over(H)}^(rec) at the operational location of the receivers is calculated.The at least one parameter set may include the at least the six unknownparameters associated with the vector parameters {right arrow over(I)}_(i) and {right arrow over (r)}_(i), and can be calculated bysolving the system of equations defined by equations (2) through (5)described above. Often, this system of equations will not have a uniquesolution, and solving the system of equations yields a plurality ofparameter sets. A forward model magnetic field {right arrow over(H)}^(for) is determined for each parameter set. The forward modelmagnetic fields {right arrow over (H)}^(for) can be determined bysolving equation (2) above. Next, for each forward model {right arrowover (H)}^(for) determined, the norm of the difference between thereceived magnetic field {right arrow over (H)}^(rec) and the forwardmodel magnetic field {right arrow over (H)}^(for), and the parameter setproducing the minimum value is selected. For example, the parameter setthat satisfies min{∥{right arrow over (H)}^(rec)−{right arrow over(H)}^(for)∥} for the estimated number of wellbores N^(pipe) is selected.

Next, the procedure 100 proceeds to step 110, where the norm of thedifference between the received magnetic field {right arrow over(H)}^(rec) and forward model magnetic {right arrow over (H)}^(for) forthe selected parameter set, i.e., min{∥{right arrow over(H)}^(rec)−{right arrow over (H)}^(for)∥} is compared to a predeterminedmisfit threshold Th^(msifit). The predetermined misfit thresholdTh^(msifit) can be based on the particular properties of the rangingtool 26, accuracy requirements, noise conditions, etc., and can bepredetermined by one skilled in the art to indicate when an acceptablesolution is found. Where min{∥{right arrow over (H)}^(rec)−{right arrowover (H)}^(for)∥} for the estimated number of wellbores N^(pipe) is notless than the predetermined misfit threshold Th^(msifit), the procedure100 proceeds to step 112 where the estimated number of wellboresN^(pipe) is incrementally increased. For example, the estimated numberof wellbores N^(pipe) is increased by one (1).

Next, in step 114, the increased estimated number of wellbores N^(pipe)is compared to the maximum number of wellbores N^(max) that arelocatable by the system 50. The maximum number of wellbores N^(max) canbe a function of the number of receivers N^(rec) provided on the rangingtool 26 as indicated above with reference to equations (7) and (9). Ifthe increased estimated number of wellbores N^(pipe) is greater than themaximum number of wellbores N^(max), it can be concluded that no validsolutions exist using the procedure 100, and the procedure 100 proceedsto step 116 where the procedure 100 ends.

If the increased estimated number of wellbores N^(pipe) is not greaterthan the maximum number of wellbores N^(max), the procedure 100 returnsto step 108 where the parameter set satisfying min{∥{right arrow over(H)}^(rec)−{right arrow over (H)}^(for)∥} for the increased estimatednumber of wellbores N^(pipe) is selected. The steps 108 through 114 canbe repeated until it is determined in step 110 that the differencebetween the received magnetic field {right arrow over (H)}^(rec) and theforward model magnetic field {right arrow over (H)}^(for) determined forthe selected parameter set is less than the predetermined misfitthreshold T^(misfit). Then, it can be concluded that the selectedparameter set represents a valid solution, and the procedure 100proceeds to optional step 118. In step 118, the individual contributionsof each of the N^(pipe) wellbores represented in the selected parameterset are calculated and compared to the predetermined field thresholdTh^(field). If any of the individual contributions are determined to benegligible, (less than the predetermined field threshold Th^(field)) thewellbores making the negligible contributions can be discarded ordisregarded. In some example embodiments, the individual contributionsmay be extracted from the calculations performed in step 108 forcalculating the forward model magnetic field {right arrow over(H)}^(for). In some instances, step 118 can result in each of theN^(pipe) wellbores being disregarded. For example, each individualwellbore can make a contribution that is less than the predeterminedfield threshold Th^(field), while the combined is greater than thepredetermined field threshold Th^(field).

Then the procedure 100 can proceed to step 120 where the non-negligiblesolutions can be returned. The solutions returned can include, forexample, the vector parameter {right arrow over (r)}_(i) and/or thevector parameter {right arrow over (I)}_(i) for each wellbore making anon-negligible contribution. In some embodiments, parameters such as thecurrent magnitude, azimuth angle and the orientation of the wellborescan be measured or determined from the selected parameter set by thedata processing unit 64, and these parameters can be returned assolutions. In some exemplary embodiments, the solutions can be returnedto the visualizing unit 60 for evaluation by a user, or to any othercomponent of system 50.

Referring now to FIG. 5B, and with continued reference to FIGS. 2 and 4,an exemplary embodiment of an operational procedure 200 for forming awellbore in a geologic formation “G” is described. The procedure 200begins at step 202 where, a first wellbore 12 is identified extendingalong a first path. The first wellbore 12 can be an existing wellboreidentified in a crowded field, or the first wellbore 12 can be drilledusing any drilling method recognized in the art. A first electricalcurrent I_(eff) ₁ can then be induced in the first wellbore and a secondcurrent I_(eff) ₂ can be induced in the second wellbore 14 (step 204).In some embodiments, the first and second electrical currents I_(eff) ₁and I_(eff) ₂ can be excited remotely, e.g., with transmitters 52 ofranging tool 26. Thus, the second current I_(eff) ₂ can be excited inthe second wellbore 14 without expressly identifying the location orverifying the existence of the second wellbore 14. In other embodiments,the first and second currents I_(eff) ₁ and I_(eff) ₂ can be directlyexcited by current source 32 (FIG. 1) at the surface location “S.” Inone or more embodiments, the first and second currents I_(eff) ₁ andI_(eff) ₂ can be excited at a different point within procedure 200, andthe first and second currents I_(eff) ₁ and I_(eff) ₂ can be maintainedcontinuously or continually throughout the procedure 200.

At step 206, a third path can be defined having a predeterminedrelationship to the first path is defined. In one or more exemplaryembodiments, the third path can be generally parallel to the first path,e.g., when the first and third wellbores comprise an SAGD pair. In otherembodiments, the predetermined relationship can be an intersectingrelationship where the third wellbore intersects the first wellbore at apredetermined or true depth, a lateral branching relationship where thethird wellbore branches from the first wellbore at a predeterminedlocation. A number of wellbores N^(pipe) nearby at least a portion ofthe third path can be estimated to determine a sufficient number ofreceivers N^(rec) for locating the estimated number of nearby wellboresN^(pipe). The sufficient number of receivers can be determined, e.g.,from equations (7) and (9) above, and in some embodiments is at leastfive (5) times the estimated number of nearby wellbores N^(pipe). Aranging tool 26 including the sufficient number of receivers 54 can beprovided in a drill string 18 (FIG. 1A).

Next in step 208, the drill string 18 can be employed to begin drillingthe third wellbore 16 along the third path. When the ranging tool 26 isdisposed at a first operational position within the third wellbore 16,the received magnetic field {right arrow over (H)}^(rec) can be detectedwith the receivers 54, and the received magnetic field {right arrow over(H)}^(rec) can be communicated to the data processing unit 64 (step210).

Next, at step 212, the data processing unit 64 can perform procedure 100(FIG. 5A) to determine a number of wellbores N^(pipe) contributing tothe received magnetic field {right arrow over (H)}^(rec) at the firstoperational position. The data processing unit 64 can thereby determinethe location of at least the first wellbore 12 with respect to the thirdwellbore 16. The location of the first wellbore 12 is returned, and canbe used to adjust a direction of drilling to maintain the path of thethird wellbore 16 in the predetermined relationship with the first pathof the first wellbore 12 (step 214).

Next the procedure 200 returns to step 210 where the ranging tool 26 canagain receive a received magnetic field {right arrow over (H)}^(rec)from a second operational position along the third path. The procedure200 can repeat steps 210 through 214 until the third wellbore 16 iscomplete, and close tolerances can be maintained even when the number ofwellbores N^(pipe) making a non-negligible contribution to the receivedmagnetic field changes along the third path.

2. Example Implementation

Referring now to FIGS. 6 through 9B, one example of a mathematicallysimulated a two-wellbore system 300 is illustrated. A first wellbore 302(Pipe 1) and a second wellbore 304 (Pipe 2) are simulated in a commonplane designated by axes x and y. Currents I₁ and I₂ are simulated ingenerally opposite directions along the first and second paths definedby the first and second wellbores 302, 304 as illustrated by arrows 306and 308. The simulated currents I₁ and I₂ each have a magnitude 1 Amp. Athird path 310 is also illustrated between the first and secondwellbores 302, 304 along which ranging tool 26 (FIG. 7) is moved.

As illustrated in FIG. 7, the ranging tool 26 is simulated as including4 tri-axial receivers 312 a, 312 b, 312 c and 312 d disposedsymmetrically around reference point “P.” The tri-axial receivers 312 a,312 c are separated along the y-axis by 20 ft., and the tri-axialreceivers 312 b, 312 d are separated along the x-axis by 20 ft. Thereference point “P” at the center of the tri-axial receivers 312 a, 312b, 312 c and 312 d was simulated to move along the third path 310illustrated in FIG. 6.

At a plurality of operational locations along the third path 310, aforward model magnetic field {right arrow over (H)}^(for) was calculatedinduced by the first and second currents I₁ and I₂ for each of thereceivers 312 a, 312 b, 312 c and 312 d. To simulate a received magneticfield {right arrow over (H)}^(rec), a one percent 1% multiplicativeerror with uniform distribution was added to the calculated forwardmodel magnetic field {right arrow over (H)}^(for) for each of thereceivers 312 a, 312 b, 312 c and 312 d at each of the operationallocations along the third path 310. Once the received magnetic field{right arrow over (H)}^(rec) was simulated, the system of equationsillustrated by equations (2) through (5) were solved as described abovewith reference of to step 108 of procedure 100 (FIG. 5A). Since thenumber of simulated wellbores was known, the variable N^(pipe) wasdefined as two (2), and the iterative process of steps 110, 112, and 114were not necessary for determining the number of nearby wellbores.

The parameter set satisfying min{∥{right arrow over (H)}^(rec)−{rightarrow over (H)}^(for)∥} for an N^(pipe) of two (2) wellbores wasdetermined and the locations of the first and second wellbores 302, 304were extracted therefrom for each of the operational locations. Theextracted locations are plotted in FIG. 8 along the simulated first andsecond paths for the first and second wellbores 302, 304. The extractedlocations for the first wellbore 302 are illustrated as asterisks andthe extracted locations for the second wellbore 304 are illustrated asdiamonds. As illustrated in FIG. 8, the extracted locations are moreaccurate where the third path 310 of the ranging tool 26 (is relativelyclose to the first and second wellbores 302, 304 (toward the right ofthe graph). In this example, the accuracy is particularly high where theranging tool 26 is closer than a radial distance “R” of about 61 meters(about 200 feet). This range is relatively important for well avoidanceand intersection purposes.

As illustrated in FIGS. 9A and 9B, currents were also extracted from theparameter set found to satisfy min{∥{right arrow over (H)}^(rec)−{rightarrow over (H)}^(for)∥}. Again, in FIGS. 9A and 9B, the x-axisrepresents the position of the respective wellbores along the x-axisdirection. The y-axis represents the magnitude of the extractedcurrents. The current magnitudes illustrated are closest to the 1 Ampmagnitude of the first and second currents I₁ and I₂ (FIG. 6) where theranging tool 26 is simulated to be closest to the simulated wellbores(toward the right of the graphs). Again, the accuracy is illustrated tobe relatively high in a range where accuracy is relatively important forwell avoidance and well intersection purposes.

3. Aspects of the Disclosure

In one aspect of the disclosure, a method of locating multiple wellboresincludes (a) exciting a first electrical current in a first wellbore,(b) exciting a second electrical current in a second wellbore, (c)disposing a ranging tool at a remote location with respect to the firstand second wellbores, (d) receiving and detecting a magnetic field atthe remote location with receivers provided on the ranging tool, and (e)measuring at least one wellbore parameter of each of the first wellboreand the second wellbore from the magnetic field received by the rangingtool.

In some exemplary embodiments, the at least one wellbore parameterincludes at least one of the group consisting of the first electricalcurrent, the second electrical current, a distance of either of thefirst and second wellbores from the receivers, azimuth angle of eitherof the first and second wellbores, and an orientation of first andsecond wellbores. The at least one wellbore parameter can include amagnitude of least one of the first current and the second current.

In one or more exemplary embodiments, the method further includesdetermining that the received magnetic field is greater than apredetermined field threshold. The method may also include calculating acontribution of each of the first and second to wellbores to thereceived magnetic field, and determining that the contribution of atleast one of the first and second wellbores is greater than thepredetermined field threshold.

In some exemplary embodiments, the method further includes determiningfrom the received magnetic field a number of wellbores contributing tothe received magnetic field. The method may also include determiningfrom the received magnetic field that the number of wellborescontributing to the magnetic field received by the ranging tool isgreater than two and measuring from the received magnetic field at leastone wellbore parameter of at least one wellbore other than the firstwellbore and the second wellbore. The method can include determiningthat the contribution of the at least one wellbore other than the firstwellbore and the second wellbore to the received magnetic field isgreater than a predetermined threshold value.

In some exemplary embodiments, determining the number of wellborescontributing to the magnetic field may include (a) estimating the numberof wellbores to define an estimated number of wellbores, (b) calculatingat least one parameter set for the estimated number of wellbores whichwould produce the received magnetic field, (c) determining a forwardmodel magnetic field for each parameter set calculated for the estimatednumber of wellbores, (d) selecting the parameter set of the at least oneparameter set for which a difference between the received magnetic fieldand the forward model magnetic field is the least, and (e) determiningthat the difference between the received magnetic field and forwardmodel magnetic field for the selected parameter set is less than apredetermined misfit threshold.

In one or more embodiments, the method may further include determiningthat the difference between the received magnetic field and forwardmodel magnetic field is not less than the predetermined misfit thresholdand incrementally increasing the estimated number of wellbores. Themethod may also include determining that the incrementally increasedestimated number of wellbores is not greater than a maximum number ofwellbores, wherein the maximum number of wellbores is based on a numberof receivers provided on the ranging tool. In some exemplary embodimentsthe method further includes determining that a magnitude of the receivedmagnetic field is greater than a predetermined field threshold, whereinthe field threshold is selected based on an accuracy of the receivers indetecting a magnetic field strength. The method may include selectingthe predetermined misfit threshold based on properties of the rangingtool and accuracy requirements for the measurement of the at least onewellbore parameter of each of the first wellbore and the secondwellbore.

In some exemplary embodiments, disposing the ranging tool at a remotelocation includes deploying the ranging tool on a drill string within athird wellbore, wherein measuring at least one wellbore parametercomprises measuring a distance of at least one of the first and secondwellbores from the receivers, and wherein the method further comprisesdirectionally drilling the third wellbore in a predeterminedrelationship with respect to at least one of the first and secondwellbores. The predetermined relationship may include at least one of aparallel relationship, a non-intersecting relationship, an intersectingrelationship, and a laterally branching relationship.

In some exemplary embodiments, the first and second currents are excitedat the same frequency, and the received magnetic field is received by anarray of single axis magnetometers located in a third wellbore. In someembodiments, the array of single axis magnetometers may include eight(8) magnetometers, and the eight (8) magnetometers may be staggeredalong an axis of the ranging tool. In other exemplary embodiments, anarray of single axis magnetometers is provided for receiving thereceived magnetic field, where the number of single axis magnetometersin the array is at least two (2) less than three times an estimatednumber of nearby wellbores (i.e., N^(rec)≥N^(pipe)−2) at the remotelocation with receivers provided on the ranging tool.

In some exemplary embodiments, the first and second wellbores are aninjector and producer respectively of an SAGD pair. In some embodiments,the first wellbore is a producer of a first SAGD pair, and the secondwellbore is a producer of a second SAGD pair. In some embodiments, thefirst wellbore is an injector of a first SAGD pair and the secondwellbore is a producer of a second SAGD pair. In some embodiments, thefirst wellbore is an injector of a first SAGD pair and the secondwellbore is an injector of a second SAGD pair. In some embodiments, thefirst wellbore is a producer of an SAGD pair, and the second wellbore isa lateral wellbore branching from the first wellbore.

According to another aspect of the disclosure a method of forming awellbore in a geologic formation includes (a) identifying a firstwellbore in the geologic formation along a first path, (b) exciting afirst electrical current along the first path in the first wellbore anda second electrical current along a second path in a second wellbore,(c) drilling a third wellbore along a third path having a predeterminedrelationship to the first path, (d) detecting a received magnetic fieldin the third wellbore, (e) determining from the received magnetic fielda number of wellbores contributing to the received magnetic field, (f)determining a location of the first wellbore based on the number ofwellbores determined to be contributing to the received magnetic field,and (g) adjusting a direction of drilling of the second wellbore tomaintain the third path in the predetermined relationship to the firstpath.

In one or more exemplary embodiments, determining the number ofwellbores contributing to the magnetic field includes (a) estimating thenumber of wellbores to define an estimated number of wellbores, (b)calculating at least one parameter set for the estimated number ofwellbores which would produce the received magnetic field, (c)determining a forward model magnetic field for each parameter setcalculated for the estimated number of wellbores, (d) selecting theparameter set of the at least one parameter set for which a differencebetween the received magnetic field and the forward model magnetic fieldis the least, and (c) determining that the difference between thereceived magnetic field and forward model magnetic field for theselected parameter set is less than a predetermined misfit threshold. Insome embodiments, the determined location of the first wellbore is aparameter in the selected parameter set.

In some exemplary embodiments, the method further includes estimatingthe number of wellbores to be determined from the received magneticfield and deploying a ranging tool having N^(rec) single axis receiverswhere N^(rec) is at least five times greater than an the estimatednumber of wellbores. In some exemplary embodiments, the predeterminedrelationship is a generally parallel relationship wherein one of thefirst and third wellbores is shallower than the other of the first andthird wellbores such that the first and second wellbores together definean SAGD pair.

In another aspect of the disclosure, a system for locating multiplewellbores include (a) a non-transitory memory having a set ofinstructions thereon, wherein the instructions include instructions foraccepting a received magnetic field as input, instructions fordetermining from the received magnetic field a number of wellborescontributing to the received magnetic field and instructions fordetermining at least one parameter of each wellbore determined to becontributing to the received magnetic field; and a processor forexecuting the set instructions.

In some exemplary embodiments, the instructions for determining thenumber of wellbores contributing to the received magnetic field mayinclude instructions for (a) estimating the number of wellborescontributing to the received magnetic field to define an estimatednumber of wellbores, (b) calculating at least one parameter set for theestimated number of wellbores which would produce the received magneticfield, (c) determining a forward model magnetic field for each parameterset calculated for the estimated number of wellbores, (e) selecting theparameter set of the at least one parameter set for which a differencebetween the received magnetic field and the forward model magnetic fieldis the least, and (f) determining whether the difference between thereceived magnetic field and forward model magnetic field for theselected parameter set is less than a predetermined misfit threshold.

In one or more exemplary embodiments, the system further includes adrill string and ranging tool carried by the drill string. In someembodiments, the ranging tool may include a receiver operable to detectand measure the received magnetic field, and the ranging tool may becommunicatively coupled to the processor. In some exemplary embodiments,the ranging tool may further include a transmitter operable to generatean electromagnetic probe signal that causes current to flow inelectrically conductive bodies exterior to the transmitters to therebygenerate the received magnetic field. Also, in some exemplaryembodiments, the system may further include a transmitter deployedindependently of the ranging tool into a first wellbore of a SAGD pairof wellbores, and the drill string may be deployed into a secondwellbore of the SAGD pair.

Moreover, any of the methods described herein may be embodied within asystem including electronic processing circuitry to implement any of themethods, or a in a computer-program product including instructionswhich, when executed by at least one processor, causes the processor toperform any of the methods described herein.

The Abstract of the disclosure is solely for providing the United StatesPatent and Trademark Office and the public at large with a way by whichto determine quickly from a cursory reading the nature and gist oftechnical disclosure, and it represents solely one or more embodiments.

While various embodiments have been illustrated in detail, thedisclosure is not limited to the embodiments shown. Modifications andadaptations of the above embodiments may occur to those skilled in theart. Such modifications and adaptations are in the spirit and scope ofthe disclosure.

1-13. (canceled)
 14. A method of forming a wellbore in a geologicformation, the method comprising: identifying a first wellbore in thegeologic formation along a first path; exciting a first electricalcurrent along the first path in the first wellbore and a secondelectrical current along a second path in a second wellbore; drilling athird wellbore along a third path having a predetermined relationship tothe first path; detecting a received magnetic field in the thirdwellbore; determining from the received magnetic field an unknown numberof wellbores contributing to the received magnetic field; determining alocation of the first wellbore based on the number of wellboresdetermined to be contributing to the received magnetic field; andadjusting a direction of drilling of the second wellbore to maintain thethird path in the predetermined relationship to the first path.
 15. Themethod of claim 14, wherein determining the number of wellborescontributing to the magnetic field comprises: estimating the number ofwellbores to define an estimated number of wellbores; calculating atleast one parameter set for the estimated number of wellbores whichwould produce the received magnetic field; determining a forward modelmagnetic field for each parameter set calculated for the estimatednumber of wellbores; selecting the parameter set of the at least oneparameter set for which a difference between the received magnetic fieldand the forward model magnetic field is the least; and determining thatthe difference between the received magnetic field and forward modelmagnetic field for the selected parameter set is less than apredetermined misfit threshold.
 16. The method of claim 15, wherein thedetermined location of the first wellbore is a parameter in the selectedparameter set.
 17. The method of claim 14, further comprising:estimating the number of wellbores to be determined from the receivedmagnetic field; and deploying a ranging tool having N^(rec) single axisreceivers where N^(rec) is at least five times ater than an theestimated number of wellbores.
 18. The method of claim 14, wherein thepredetermined relationship is a generally parallel relationship whereinone of the first and third wellbores is shallower than the other of thefirst and third wellbores such that the first and second wellborestogether define a SAGD pair.
 19. A system for locating multiplewellbores, comprising: a non-transitory memory having a set ofinstructions thereon, the instructions including instructions foraccepting a received magnetic field as input, instructions fordetermining from the received magnetic field an unknown number ofwellbores contributing to the received magnetic field and instructionsfor determining at least one parameter of each wellbore determined to becontributing to the received magnetic field; and a processor forexecuting the set instructions.
 20. The system of claim 19, wherein theset of instructions for determining the number of wellbores contributingto the received magnetic field includes instructions for: estimating thenumber of wellbores contributing to the received magnetic field todefine an estimated number of wellbores; calculating at least oneparameter set for the estimated number of wellbores which would producethe received magnetic field; determining a forward model magnetic fieldfor each parameter set calculated for the estimated number of wellbores;selecting the parameter set of the at least one parameter set for whicha difference between the received magnetic field and the forward modelmagnetic field is the least; and determining whether the differencebetween the received magnetic field and forward model magnetic field forthe selected parameter set is less than a predetermined misfitthreshold.
 21. The system of claim 19, further comprising a drill stringand ranging tool carried by the drill string, wherein ranging toolcomprises a receiver operable to detect and measure the receivedmagnetic field, and wherein the ranging tool is communicatively coupledto the processor.
 22. The system of claim 21, wherein the ranging toolfurther comprises a transmitter operable to generate an electromagneticprobe signal that causes current to flow in electrically conductivebodies exterior to the transmitters to thereby generate the receivedmagnetic field.
 23. The system of claim 21, further comprising atransmitter deployed independently of the ranging tool into a firstwellbore of a SAGD pair of wellbores, and wherein the drill string isdeployed into a second wellbore of the SAGD pair.
 24. The method ofclaim 14, wherein determining the unknown number of wellborescontributing to the received magnetic field includes determining thenumber of nearby wellbores producing a contribution to the receivedmagnetic field that is greater than a field threshold.
 25. The method ofclaim 14, wherein exciting the first electrical current and the secondelectrical current includes exciting the first electrical current andthe second electrical current the same frequency.
 26. The method ofclaim 14, wherein exciting the first electrical current and the secondelectrical current includes concurrently exciting the first electricalcurrent and the second electrical current such that the receivedmagnetic field includes a first contribution from a first magnetic fieldproduced by the first electrical current and a second contribution froma second magnetic field produce by the second electrical current. 27.The method of claim 1, further comprising measuring each of a trajectoryand a position of each first and second wellbores.
 28. The system ofclaim 19 wherein the set of instructions for determining the number ofwellbores contributing to the received magnetic field includesinstructions for determining a contribution to the receive magneticfield made by each of the first and second electrical currents.
 29. Thesystem of claim 20 wherein the set of instructions for calculating atleast one parameter set for the estimated number of wellbores whichwould produce the received magnetic field includes instructions fordetermining each of a trajectory and a position of each of the first andsecond wellbores.